In the drilling of oil wells such as oil and gas wells, cores are taken of the subterranean formation through which the wells are drilled and various characteristics of the cores, or core samples, are determined for the purpose of identifying different fluids in the formation, estimating the quantity of each fluid in the formation, the ease of flow through the formation, etc. Such core samples are also taken from producing reservoirs and characteristics of the core samples are determined for the purpose of estimating particular fluid quantities, predicting production rates, etc.
Two of such characteristics are the capillary pressure drainage and imbibition of a core sample. Such capillary characteristics, as well as measurements thereof, are set forth in Physical Principles of Oil Production, McGraw Hill Book Company, 1949, pp 304-311, by Morris Muskat. The importance of determining capillary characteristics pertains to the fluid distribution in a hydrocarbon-bearing (i.e., oil or gas) formation prior to its production. Capillary pressure is the pressure required to cause a fluid to displace from the openings in a porous rock another fluid with which it is not miscible and is dependent on the size of the openings, the interfacial tension between the two fluids, and the contact angle of the system.
Characteristic measurements on a core sample of a porous rock during capillary pressure drainage have been described in U.S. Pat. Nos. 4,907,448 and 4,926,128 to Givens and U.S. Pat. No. 4,924,187 to Sprunt et al. While centrifuge techniques have been used for measurements taken during capillary pressure imbibition, corrections must be applied to such centrifuge measurements for the pressure gradient within the core sample. On the other hand, the measurements taken during capillary pressure drainage described in the aforementioned patents provides a direct measurement of capillary pressure curves (i.e., water saturation versus capillary pressure).
It is a specific object of the present invention to provide a method for conducting capillary pressure drainage and imbibition of a core sample of a porous rock that provides for direct measurement of both capillary drainage and imbibition pressure curves without the limitations of centrifuge techniques on the imbibition measurements.